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APPENDIX                                    235

A simpler method is to replace the alcohol hydwxyl by halo-
gen by the action of hydracid (HC1, HBr, HI),'
C,Hf)OII + HC1 = C2n0Cl + H,0.
Or by that of the phosphorus compound (POL, PBr3, PI3),
3C,H,OH + PCL = 3C2II5Cl + P(OII),.
The preparation of ethyl bromide may be taken as an ex-
ample of the first method, in which the hydracid is liberated by
the reaction,
KBr + II2SO4 = II Br + KIIS04.
A further example is that of isopropyl iodide: see Prep. 31,
p. no, in which the hydriodic acid is obtained by the action of
water on phosphorus iodide,
PL, + 3HoO = 3III + P(OII)3.
The action of II Cl is much more sluggish than that of HBr
or III, and in the preparation of ethyl chloride a dehydrating
agent (ZnCL) is usually added to the alcohol, which is kept
boiling whilst the HC1 gas is passed in. In the case of poly-
hydric alcohols, all the hydroxyl groups cannot be replaced by
Cl by the action of HC1. Glycol gives ethylene chlorhydrin and
glycerol yields the<lichlorhydrin (see Prep. 32, p. HI). The use
of IM>r;j, PI3 does not necessitate the previous preparation of
these substances. Amorphous phosphorus is mixed with the
alcohol, and bromine or iodine added as in the preparation of
methyl iodide (see Prep. 6, p. 68). PC15 or PC13 will always
replace OH by chlorine in all hydroxy-compounds, including
phenols, on which HCl does not act.
The alkyl halides are utilised in a variety of reactions,
examples of which are given, ethyl iodide being taken as the
type.
r. Agucoits potash or water with metallic oxide (Ag\,O, PbO)
yields the alcohol (see Prep. 87, p. 195),
Coiirj + KOII = c.2n,oir + KI.
2.   Alcoholic potash gives an olefine,
CoHr,I + KOII = C2H.t + KI + HoO.
3.   Sodium alcoholate gives an ether,
ji = C2HBOC.JIS + NaL